Brad, There is old school that has existed for a long time and them there are new things, like thumbnail dies. Here is a picture of some of my thumbnail dies
These dies are used with a reciprocating machine, pullmax, Nibbler or other types. These dies will shrink metal quickly by forming a tuck in the metal and then as the metal is pulled back out of the machine it will drive the metal back into itself causing the metal to shrink. It does the same thing as using a tucking fork and hammer just faster.
Hope this helps you understand thumbnail dies.